Multi-stage gate turn-off with dynamic timing

ABSTRACT

A turn-off circuit for a semiconductor switch includes an element having a variable resistance coupled to a control input of the semiconductor switch, a circuit for generating a control-input reference signal, and a control circuit coupled to adjust a resistance of the element having a variable resistance in response to the control-input reference signal in a closed control loop in order to turn off the semiconductor switch.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/744,862, filed Jun. 19, 2015, now pending, which claims the benefitof U.S. Provisional Application No. 62/022,304, filed Jul. 9, 2014,entitled “MULTI-STAGE GATE TURN-OFF WITH DYNAMIC TIMING.” U.S. patentapplication Ser. No. 14/744,862 and U.S. patent application Ser. No.62/022,304 are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to turn-off circuits for a semiconductorswitch, systems comprising a turn-off circuit for a semiconductor switchand an active clamping circuit and methods for turning off asemiconductor switch.

BACKGROUND

Several methods are known for turning off a semiconductor switch (and inparticular a power semiconductor switch) in the event of a short-circuitstate and/or overcurrent state. In one example, a control input (forexample a gate terminal) of the semiconductor switch can be coupled to afirst and second resistance, wherein the first resistance is arranged soas to couple the control input of the semiconductor switch, duringnormal operation, to a reference potential (for example an emittervoltage of the semiconductor switch) and thus to turn off thesemiconductor switch. In the event of a short circuit and/or anovercurrent event, a second, greater resistance can now be coupledbetween the control input of the semiconductor switch and the referencepotential. As a result, a current with which the control input of thesemiconductor switch is discharged with is reduced. The voltage at thecontrol input of the semiconductor switch is reduced less quickly thanduring normal operation. It is thus possible to prevent an excessivelyabrupt turn-off operation which may result in a hazardous overvoltagecondition in the semiconductor switch determined by parasiticinductances across a load at the output of the semiconductor switch.

SUMMARY OF THE INVENTION

A first turn-off circuit for a semiconductor switch comprises an elementhaving a variable resistance, said element being coupled to a controlinput of the semiconductor switch, a circuit for generating acontrol-input reference signal, and a control circuit, which is designedto adjust a resistance of the element having a variable resistance inresponse to the control-input reference signal in a closed control loopin order to turn off the semiconductor switch.

An adjustment of the element having a variable resistance in a closedcontrol loop makes it possible to provide a dynamic turn-off circuitwhich achieves satisfactory results with various different semiconductorswitches. Since a current through the control input of the semiconductorswitch (for example through the gate input) can be adjusted via theadjusted resistance of the element having a variable resistance, it ispossible for the turn-off circuit to be matched dynamically to therespective semiconductor switch. In some turn-off circuits from theprior references, different components need to be used for differentsemiconductor switches in order to ensure a satisfactory profile of thecontrol input voltage. For example, it can be indicated in the aboveexample to use resistances of different sizes for different types ofsemiconductor switches. If this matching does not occur, it may arisethat a short-circuit state lasts for longer (if the resistance is higherthan is necessary) or the voltage across a load at the output of thesemiconductor switch is not reduced to the sufficient extent. Other morecomplex turn-off circuits require a higher number of and/or expensivecomponents.

A second turn-off circuit for a semiconductor switch comprises anelement having a variable resistance, said element being coupled to acontrol input of the semiconductor switch, a detection circuit, which isdesigned to detect an end of a Miller plateau in a control input voltageor in a corresponding control input current of the semiconductor switch,and a control circuit, which is designed to control a resistance valueof the element having a variable resistance in such a way that a voltagewhich is present at the control input of the semiconductor switch isreduced after the end of a Miller plateau at a predetermined rate.

By coupling a turn-off profile of the semiconductor switch to thedetection of an end of a Miller plateau, the second turn-off circuit canlikewise be matched dynamically to different semiconductor switches. TheMiller plateau (i.e. a region in which substantially the total currentthrough the control input contributes to the charging of a parasiticcapacitance between the control input and the drain, collector or anodeinput of the semiconductor switch so that a voltage between the controlinput of the semiconductor switch and a source, emitter or cathode inputof the semiconductor switch remains substantially constant) can bewithin a similar voltage range for different semiconductor switches (inmany cases even for a wide range of temperature and process parameters).For example, the Miller plateau can be between 9.5 volts and 11.5 voltsin many IGBTs. Furthermore, an end of the Miller plateau can indicate atime at which a drain, collector or anode current of a semiconductorswitch is reduced to a safe level in the event of a short circuit and/oran overcurrent and, on the other hand, continuance of the turn-offoperation may not result in high overvoltages across an output of thesemiconductor switch. Thus, an advantageous, dynamic switching point fora change in a turn-off characteristic of a semiconductor switch can beselected.

In a first embodiment, a turn-off circuit for a semiconductor switchincludes an element having a variable resistance, said element beingcoupled to a control input of the semiconductor switch, a circuit forgenerating a control-input reference signal, and a control circuit,which is designed to adjust a resistance of the element having avariable resistance in response to the control-input reference signal ina closed control loop in order to turn off the semiconductor switch.

This turn-off circuit can have one or more of the following features.For example, the element having a variable resistance is a semiconductorswitch. In a further example, the element having a variable resistanceis a MOSFET semiconductor switch the element having a variableresistance is a MOSFET semiconductor switch. Also, a variable resistanceof the element having a variable resistance is formed between a drain,anode or collector terminal and a source, cathode or emitter terminal ofthe semiconductor switch. Further, the element having a variableresistance is coupled in series with a further resistance between thecontrol input of the semiconductor switch and a reference potential. Forexample, the control-input reference signal has a first drop at a firstrate, a region with a substantially constant signal level, and a seconddrop at a second rate. As another example, the second rate is higherthan the first rate. Further, the first and second rates are temporallyvariable. In a further example, the turn-off circuit further includes adetection circuit which is designed to detect an end of a Miller plateauin a control input voltage or in a control input current of thesemiconductor switch, wherein the circuit for generating a control-inputreference signal is designed to reduce a level of the control-inputreference signal in response to a detection of an end of a Millerplateau at a predetermined rate. Also, an end of a Miller plateau isdetected on the basis of a voltage at the control input of the elementhaving a variable resistance. Further, an end of a Miller plateau isdetected when the voltage at the control input of the element having avariable resistance falls below a determined signal level. For example,the predetermined signal level is in a range of from 50% to 150% of anexpected gate threshold voltage of the element having a variableresistance. In another example, the predetermined signal level is in arange of between 0.3 and 2 volts. Also, the predetermined signal levelis determined by means of a reference current and by means of a secondelement based on the same technology as the element having a variableresistance. In some cases, an area or a gate width of the second elementis K times an area or a gate width of the element having a variableresistance, and wherein the reference current is selected such that itis K times a threshold value of the output current of the element havinga variable resistance which is designed for the end of the Millerplateau. For example, the reference current is coupled to the controlinput, in particular to the gate of the second element having a variableresistance and to the output, in particular to the drain, of the secondelement having a variable resistance. Also, the reference current isselected to be less than 100 microamperes and K is selected to be lessthan 1%. In some cases, the level of the control-input reference signalprior to an end of a Miller plateau being reached is substantiallyconstant, and, in response to the detection of the end of the Millerplateau, the signal level of the control-input reference signal isreduced at the predetermined rate. Further, the turn-off circuitfurthermore comprises a circuit for detecting a voltage at the controlinput of the semiconductor switch. As in another example, the controlcircuit is designed to adjust the resistance of the element having avariable resistance in response to the control-input reference signaland the voltage at the control input of the semiconductor switch. Also,the control circuit comprises a first comparison circuit in order tocompare the control-input reference signal with the voltage at thecontrol input of the semiconductor switch. For example a circuit forgenerating a control signal for the element having a variable resistancein response to an output of the first comparison circuit. For example,the circuit for generating a control signal comprises a secondcomparison circuit, which is designed to generate the control signal forthe element having a variable resistance in response to a comparison ofthe output of the first comparison circuit with a fault signal, whichindicates a fault state of the semiconductor switch. Also, the turn-offcircuit is designed to receive a fault signal, which indicates a faultstate of the semiconductor switch. Further, the fault state of thesemiconductor switch is a short-circuit state and/or an overcurrentstate. In another example, the semiconductor switch is a powersemiconductor switch. As a further example, the power semiconductorswitch is an IGBT, an IEGT, a power MOSFET or a power bipolartransistor. In some cases, the turn-off circuit furthermore comprises anactive clamping circuit. For example, an end of a Miller plateau isdetected on the basis of a voltage across the element having a variableresistance. Also, an end of a Miller plateau is detected on the basis ofa current at the control input of the semiconductor switch. Further, anend of a Miller plateau is detected on the basis of a voltage across thesemiconductor switch. In some cases, an end of a Miller plateau isdetected on the basis of a useful current through the semiconductorswitch. For example, the control circuit is designed to detect theresistance of the element having a variable resistance on the basis ofthe control-input reference signal and the voltage across the elementhaving a variable resistance, the current at the control input of thesemiconductor switch, the voltage across the semiconductor switch or theuseful current through the semiconductor switch. In another example, thecircuit for generating a control-input reference signal comprises two ormore current sources and a capacitance, wherein the capacitance isarranged so as to be discharged from the two or more current sources inthe event of a fault. Also, the capacitance is coupled to apredetermined voltage after a turn-on operation of the semiconductorswitch. Further, a first of the two or more current sources is designedto discharge the capacitance after a time at which an end of a Millerplateau has been detected. In some cases, a second of the two or morecurrent sources is designed to discharge the capacitance up to a time atwhich a threshold voltage is present across the capacitance. Forexample, the turn-off circuit furthermore comprises a deactivationcircuit, which, in response to a deactivation signal, prevents thecontrol circuit from adjusting a resistance of the element having avariable resistance in response to the control-input reference signal.

In a second embodiment, a turn-off system includes one of the turn-offcircuits of the first embodiment and the one or more features of theturn-off circuit, an active clamping circuit, which is designed toactively increase an output voltage of a driver circuit of thesemiconductor switch to the extent that is necessary in order to keep anoutput voltage of the semiconductor switch below a determined thresholdvoltage, and a selection circuit, which, in response to a selectionsignal, activates either the turn-off circuit or the active clampingcircuit in order to turn off the semiconductor switch in the event of afault.

The turn-off system can have one or more of the following features. Forexample, an output of the active clamping circuit is coupled to acontrol input of the element having a variable resistance. Also, theactive clamping circuit reduces a voltage at the control input of theelement having a variable resistance when a voltage between the controlinput of the element having a variable resistance and a source,collector or cathode input of the semiconductor switch approaches apredetermined threshold voltage. Further, the turn-off system isdesigned in such a way that coupling of the control-input referencesignal to the control input of the element having a variable resistanceis prevented when the active clamping circuit is activated.

In a third embodiment, a turn-off circuit for a semiconductor switchincludes an element having a variable resistance, said element beingcoupled to a control input of the semiconductor switch, a detectioncircuit, which is designed to detect an end of a Miller plateau in acontrol input voltage or in a control input current of the semiconductorswitch, and a control circuit, which is designed to control a resistancevalue of the element having a variable resistance in such a way that avoltage which is present at the control input of the semiconductorswitch is reduced after the end of a Miller plateau at a predeterminedrate.

The turn-off circuit can have one or more of the following features. Forexample, an end of a Miller plateau is detected on the basis of avoltage at the control input of the element having a variableresistance. In some cases, an end of a Miller plateau is detected whenthe voltage at the control input of the element having a variableresistance falls below a determined signal level. Also, thepredetermined signal level is in a range of from 50% to 150% of anexpected gate threshold voltage of the element having a variableresistance. Further, the predetermined signal level is between 0.3 and 2volts. For example, the level of the control-input reference signalprior to an end of a Miller plateau being reached is substantiallyconstant, and, in response to the detection of the end of the Millerplateau, the signal level of the control-input reference signal isreduced.

In a fourth embodiment, a method for turning off a semiconductor switchincludes generating a control-input reference signal and adjusting aresistance of an element having a variable resistance, said elementbeing coupled to a control input of the semiconductor switch, inresponse to the control-input reference signal in a closed control loop.

In a fifth embodiment, a driver circuit to for use in a switchcontroller to control a power switch includes an on-state driver coupledto receive an on signal, wherein the on-state driver outputs a firstcontrol signal to turn ON the power switch in response to the on signaland the first control signal is substantially equal to a high threshold,an off-state driver coupled to receive an off signal, wherein theoff-state driver outputs the first control signal to turn OFF the powerswitch in response to the off signal and the first control signal issubstantially equal to a low threshold, and a soft shutdown circuit,coupled to receive the first control signal, wherein the soft shutdowncircuit regulates the first control signal in a closed loop in responseto a fault condition, wherein the soft shutdown circuit decreases thefirst control signal to a mid-threshold from the high threshold for aperiod of time and then decreases the first control signal to the lowthreshold, wherein the period of time ends in response to the end of aMiller plateau of the power switch.

The driver circuit can have one or more of the following features. Forexample, the soft shutdown circuit detects the end of the Miller plateauof the power switch when the off signal reaches a first threshold. Inanother example, the off-state driver further includes a transistor,wherein the soft shutdown circuit is coupled to receive a gate signalrepresentative of a gate current or a gate voltage of the transistor anddetects the end of the Miller plateau of the power switch when the gatesignal of the transistor reaches a first threshold. Also, the end of theMiller plateau of the power switch is detected when the gate signal ofthe transistor reaches a first threshold after a blanking time. Further,the shutdown circuit includes an amplifier coupled to receive the firstcontrol signal and a reference signal, wherein the reference signaldecreases to the mid threshold from the high threshold for a period oftime and then decreases to the low threshold in response to the faultcondition and the end of the Miller plateau of the power switch. In somecases, the end of the Miller plateau of the power switch is detectedwhen the gate signal of the transistor reaches a first threshold after ablanking time, wherein the blanking time may be end when the referencesignal is substantially equal to the mid threshold. For example, thereference signal is not substantially equal to the mid threshold for theperiod of time if there is no fault condition. As another example, thedriver circuit may receive an active clamping signal, wherein the softshutdown circuit is disabled when the active clamping signal is betweena first threshold and a second threshold. The active clamping signalincludes an additional current prior to the first control signal turningOFF the power switch. Further, the fault condition may be an overcurrentcondition for the power switch.

In a sixth embodiment, a circuit for turning off a power semiconductorswitch includes a turn-off transistor coupled to switch a signal forturning off the power semiconductor switch onto a control terminal ofthe power semiconductor switch and a feedback control loop forcontrolling a voltage on the control terminal of the power semiconductorswitch during turn-off, the feedback control loop. The feedback controlloop includes a feedback path to feedback a measurement of the voltageof the control terminal of the power semiconductor switch, a controlterminal reference voltage generator to generate a time-dependentreference voltage, an error amplifier to generate an error signalrepresentative of a difference between the voltage of the controlterminal and the time-dependent reference voltage and a forward path toconvey the error signal forward for controlling the switching of thesignal for turning off the power semiconductor switch onto the controlterminal of the power semiconductor switch by the turn-off transistor.

The circuit can have one or more of the following features. For example,the circuit further includes Miller plateau detection circuitry coupledto detect that the voltage of control terminal of the powersemiconductor switch is below a voltage level in a vicinity of theMiller plateau of the power semiconductor switch and output a signalindicative thereof. As another example, the control terminal referencevoltage generator is to increase a time rate of change of thetime-dependent reference voltage in response to the signal indicativethat the control terminal of the power semiconductor switch is below thevoltage level in the vicinity of the Miller plateau. In some cases, theMiller plateau detection circuitry comprises control terminal currentdetection circuitry coupled to detect current flow to the controlterminal of the power semiconductor switch. Also, the Miller plateaudetection circuitry comprises a voltage comparator coupled to comparethe control terminal of the power semiconductor switch with a referencevoltage. Further, the control terminal reference voltage generatorincludes first circuitry to change the time-dependent reference voltagefrom a first value at which the power semiconductor switch is on to asecond value in the vicinity of the Miller plateau of the powersemiconductor switch and second circuitry to change the time-dependentreference voltage from a third value in the vicinity of the Millervoltage of the power semiconductor switch to a fourth value at which thepower semiconductor switch is off. For example, the first circuitry isto change the time-dependent reference voltage at a time rate of changethat is less than a time rate of change at which the second circuitry tochange the reference voltage. As another example, the first circuitry isto change the time-dependent reference voltage from the first value tothe second value in between 400 nanoseconds and 4000 nanoseconds. Insome cases, the second circuitry is to change the time-dependentreference voltage from the third value to the fourth value in between100 nanoseconds and 2000 nanoseconds. In another example, the secondcircuitry is to change the time-dependent reference voltage from thethird value to the fourth value in between 10 nanoseconds and 100nanoseconds. Also, the second value is above the Miller plateau of thepower semiconductor switch and the third value is below the Millerplateau of the power semiconductor switch. Further, the control terminalreference voltage generator comprises circuitry to hold thetime-dependent reference voltage constant between the second value andthe third value. For some examples, fault detection circuitry coupled tooutput a fault signal in response to detection of a fault in currentconduction through the power semiconductor switch, wherein the controlterminal reference voltage generator is responsive to the fault signalto begin changing the reference voltage from a value at which the powersemiconductor switch is open. As another example, the fault detectioncircuitry comprises circuitry for detecting a collector-to-emittervoltage of the power semiconductor switch. Also, the signal for turningoff the power semiconductor switch is a current and the circuit furthercomprises a turn-off gate resistor. Further, the power semiconductorswitch is an IGBT. In some cases, the turn-off transistor is an NMOStransistor.

BRIEF DESCRIPTION OF THE FIGURES

Non-limiting and non-exhaustive exemplary embodiments of the inventionare described with reference to the following figures, wherein the samereference symbols relate to the same components in different figures,where not specified otherwise.

FIG. 1A shows an exemplary apparatus for providing electrical energy toa consumer, said apparatus having a control circuit for semiconductorswitches having the turn-off circuits described herein.

FIG. 1B shows exemplary signal profiles of voltages across a controlterminal of a semiconductor switch and signal profiles of a voltageacross a semiconductor switch during normal operation or in the event ofa short circuit.

FIG. 2 shows an exemplary control circuit for a semiconductor switchhaving a turn-off circuit described herein and an active clampingcircuit.

FIG. 3 shows an exemplary turn-off circuit.

FIG. 4 shows exemplary signal profiles in a system which has a controlcircuit for semiconductor switches having the turn-off circuitsdescribed herein.

FIG. 5 shows an exemplary turn-off circuit in combination with an activeclamping circuit.

FIG. 6 shows simulated signal profiles in an exemplary turn-off circuit.

FIG. 7 shows an exemplary circuit for generating a threshold voltage ina circuit for detecting an end of a Miller plateau.

DETAILED DESCRIPTION

Numerous details are given in the description below to enable afar-reaching understanding of the present invention. However, it isclear to a person skilled in the art that the specific details are notnecessary for implementing the present invention. Known apparatuses andmethods are not outlined in detail at another point in order to notunnecessarily hinder understanding of the present invention.

In the present description, a reference to “an implementation”, “aconfiguration”, “an example” or “example” means that a specific feature,structure or property which is described in conjunction with thisembodiment is included in at least one embodiment of the presentinvention. Thus, the phrases “in one implementation”, “in oneembodiment”, “one example” or “in one example” at different points inthis description do not necessarily all relate to the same embodiment orthe same example.

In addition, the specific features, structures or properties can becombined in any desired suitable combinations and/or subcombinations inone or more embodiments or examples. Particular features, structures orproperties can be included in an integrated circuit, in an electroniccircuit, in circuit logic or in other suitable components which providethe described functionality. Furthermore, reference will be made to thefact that the drawings are used for explanatory purposes for a personskilled in the art and that the drawings are not necessarily illustratedtrue to scale.

FIG. 1A shows an apparatus 100 (also referred to as power converter) forproviding electrical energy to a consumer 110, said apparatus having acontrol circuit for semiconductor switches having the turn-off circuitsdescribed herein. However, the flow of energy can also point in theother direction. In this case, the element 110 is a generating unit. Inthe other apparatuses, element 110 can operate in different operatingstates, both as a consumer and as a generating unit. In the followingtext, only an apparatus for providing energy is discussed, whichincludes all of the just mentioned cases (the energy can be provided atdifferent outputs). The apparatus comprises two power semiconductorswitches 104, 106, which are coupled together. In addition, theapparatus 100 can receive a DC input voltage 102 (U_(IN)). The apparatusis designed to transmit electrical energy, by controlling the powersemiconductor switches 104, 106, from the input to an output, to whichthe consumer 110 is connected (or in the reverse direction). In thiscase, the apparatus for providing electrical energy can control voltagelevels, current levels or a combination of the two variables, which areoutput to the consumer. In the example shown in FIG. 1A, the powersemiconductor switches 104, 106 are IGBTs.

In the text which follows, the apparatuses and methods are explainedusing the example of IGBTs. However, the turn-off apparatuses describedherein are not restricted to use with IGBTs. Instead, they can also beused in combination with other power semiconductor switches. Forexample, metal-oxide semiconductor field-effect transistors (MOSFETs),bipolar transistors (BJTs), injection-enhancement gate transistors(IEGTs) and gate turn-off thyristors (GTOs) can be used with theturn-off apparatuses described herein. The turn-off apparatusesdescribed herein can also be used with power semiconductor switcheswhich are based on gallium nitride (GaN) semiconductors or siliconcarbide (SiC) semiconductors.

A maximum nominal collector-emitter, anode-cathode or drain-sourcevoltage of a power semiconductor switch in the switched-off state can bemore than 500 V, preferably more than 2 kV.

In addition, the turn-off apparatuses described herein are notrestricted to use with power semiconductor switches. Thus, othersemiconductor switches can also be used with the turn-off apparatusesdescribed herein. The effects and advantages which are mentioned herealso occur at least partly in systems with other semiconductor switches.

Since IGBTs are discussed below, the terminals of the powersemiconductor switch are referred to as “collector”, “gate” and,“emitter”. As already explained above, the apparatuses and methods arenot restricted to IGBTs, however. In order to avoid unnecessarily longdescriptions, the designation “emitter” herein also includes theterminal of corresponding power semiconductor switches which are denotedby “source” or “cathode”. Equally, the term “collector” herein alsoincludes the terminal denoted by “drain” or “anode”, and the term “gate”denotes the terminal of corresponding power semiconductor switchesdenoted by “base”. In the text which follows, the term“collector-emitter voltage” also includes a “drain-source voltage” and a“cathode-anode voltage” and the terms “collector voltage” and “emittervoltage” also include a “drain voltage” or “anode voltage” and a “sourcevoltage” or “cathode voltage”, respectively.

The power semiconductor switches 104, 106 are each controlled by a firstand second control circuit 118, 120. Said control circuits provide afirst and a second gate-emitter driver signal 130, 132 (U_(GE1),U_(GE2)) in order to control the switching times of the first and secondIGBTs. Both control circuits 118, 120 can optionally in turn becontrolled by a system controller 114. The system controller 114 canhave an input for receiving system input signals 116. In the exampleshown in FIG. 1A, two power semiconductor switches 104, 106 with ahalf-bridge configuration are illustrated. The turn-off apparatuses canalso be used in other topologies, however. For example, a single powersemiconductor switch (for example a single IGBT) having an apparatus fordetection of a profile of a voltage across a power semiconductor switchor of a control circuit can be coupled. In other examples, in athree-phase system having six power semiconductor switches or twelvepower semiconductor switches, each of the power semiconductor switchescan have an apparatus for detecting a profile of a voltage across apower semiconductor switch.

In addition to the output of a gate-emitter driver signal, the controlcircuits 118, 120 receive signals which represent voltages which arepresent across the power semiconductor switches 104, 106. The signalsmay be voltage signals or current signals. In the example shown in FIG.1A, each control circuit 118, 120 has in each case one signal, which isrepresentative of the collector-emitter voltage and is denoted ascollector-emitter voltage signal 122, 124 (U_(CE1), U_(CE2)).

FIG. 1A shows the control circuits 118, 120 as separate controlcircuits. However, the two control circuits 118, 120 can also becombined in a single circuit. In this case, a single control circuitcontrols two power semiconductor switches 104, 106. Furthermore, thesecond gate-emitter driver signal 132 (U_(GE2)) can be an inverted firstgate-emitter driver signal 130 (U_(GE1)).

The two control circuits 118, 120 comprise one of the turn-offapparatuses described herein. In response to the establishment of ashort-circuit state and/or overcurrent state, the respective powersemiconductor switch 104, 106 can be turned off with the aid of theturn-off apparatuses described herein.

FIG. 1B shows exemplary signal profiles of voltages across a controlterminal of a semiconductor switch and signal profiles of a voltageacross a semiconductor switch during normal operation and in the eventof a short circuit. Signal profiles of a voltage 130 (U_(GE)) between agate terminal and an emitter terminal are shown in the upper half ofFIG. 1B. The voltage 130 (U_(GE)) is shown as have a first signal level(V_(ON)) and a second, different signal level (V_(OFF)). If the gateterminal is at the first signal level (V_(ON)), the semiconductor switchis turned on (for a time t_(ON) 131). Signal profiles of acollector-emitter voltage 125 across the semiconductor switch in thenormal case (on the left-hand side) and in an exemplary short-circuitand/or overcurrent case (on the right-hand side) when the semiconductorswitch is turned on are shown in the lower half of FIG. 1B. In theshort-circuit case shown, the collector-emitter voltage 125 does notdecrease rapidly to a relatively low value after turn-on (however, thereare still further short-circuit cases in which the collector-emittervoltage assumes other characteristic profiles). This can result indamage in the semiconductor switch and on the load. Therefore, thesemiconductor switch should be turned off quickly. However, if thistakes place too quickly, overvoltages on the load can occur. In order toprevent this, the turn-off circuits described herein can be used.

FIG. 2 shows an exemplary control circuit 218 for a semiconductor switchcomprising a turn-off circuit 242 described herein and an (optional)active clamping circuit 236. The control circuit receives controlcommands from a system controller 214 (said control commands in turn aregenerated in response to system input signals 216). At a driverinterface 226, these control commands are converted into control signals250 (U_(CMD)), which are transmitted to the driver circuit 228 via anisolating transformer 232. The driver circuit 228 controls asemiconductor switch 204 in response to the control signals 250(U_(CMD)) received via the isolating transformer 232. For this purpose,the driver circuit 228 is coupled to a control input (for example to agate input) of the semiconductor switch 204.

The exemplary driver circuit 228 shown in FIG. 2 has a driver for the ONstate and a driver for the OFF state of the semiconductor switch 244,246. Said drivers 244, 246 each generate the driver signal 230 (U_(GE))for the semiconductor switch 204. The two drivers 244, 246 arecontrolled via a driver signal processing unit 238, which receives thecontrol signals 250 (U_(CMD)) from the isolating transformer 232 (andconverts them into an ON signal 254 (U_(ON)) and an OFF signal 258(U_(OFF)) for the respective driver 244, 246).

A turn-off circuit 242 described herein is coupled between the driversignal processing unit 238 and the driver for the OFF state 246. Saidturn-off circuit 242 can ensure a turn-off operation in a short-circuitcase and/or an overcurrent case, in which during the turn-off operationa drop in the switch current 240 (in this example the collector-emittercurrent I_(CE)) through the semiconductor switch is not as steep asduring normal operation (a so-called “soft shutdown”). As a result, itis possible to prevent hazardous overvoltages from arising across theoutput of the semiconductor switch. Details in respect to the propertiesof various turn-off circuits are discussed in connection with FIG. 3,FIG. 4 and FIG. 5.

The turn-off circuit 242 receives the OFF signal 252 (U_(OFF)) for thedriver for the OFF state 246, a fault signal 248, which indicates thepresence of a short-circuit case and/or an overcurrent case, and asignal which is present at the control terminal of the semiconductorswitch 230 (for example a gate-emitter voltage U_(GE)). On the basis ofthese signals, the turn-off circuit 242 can dynamically control aturn-off operation of the semiconductor switch 204. In one example, aprofile of the signal which is present at the control terminal of thesemiconductor switch 230 can be adjusted in a closed control loop inorder to turn off the semiconductor switch 204. In the example shown inFIG. 2, adjustment can include the generation of a modified OFF signal256 (U_(OFF)*), which is transmitted from the turn-off circuit 242 tothe driver for the OFF state 246. This modified OFF signal 256(U_(OFF)*) can, in one example, vary a variable resistance of an elementhaving a variable resistance in the driver for the OFF state 246 andthus influence the profile of the signal which is present at the controlterminal of the semiconductor switch 230 (for example a gate-emittervoltage U_(GE)). In other words, the modified OFF signal 256 (U_(OFF)*)can be a manipulated variable of the control loop. In the example shownin FIG. 2, the signal which is present at the control terminal of thesemiconductor switch 230 (for example a gate-emitter voltage U_(GE)) isa feedback parameter of the control loop. However, other parameters canalso be used as feedback parameters.

In addition or as an alternative, the driver for the OFF state 246 canbe controlled in such a way that a voltage which is present at thecontrol input of the semiconductor switch 230 (for example agate-emitter voltage U_(GE)) is reduced after the end of a Millerplateau at a predetermined rate. In this way, the dynamic turn-offcircuit 242 can ensure a suitable “soft shutdown” for differentsemiconductor switches. In one example, an end of the Miller plateau ina control input voltage or in a control input current of thesemiconductor switch 204 can be detected on the basis of a profile of avoltage at a control input of an element having a variable resistance258 (U_(G_OFF)) in the driver for the OFF state 246.

The control circuit 218 includes a short-circuit and/or overvoltagedetection circuit 234, which generates the fault signal 248 (U_(FLT)).In one example, the short-circuit and/or overvoltage protection circuit234 can monitor a collector-emitter voltage 222 (U_(CE)) of thesemiconductor switch 204. As mentioned in connection with FIG. 1B, saidcollector-emitter voltage 222 (U_(CE)) can assume a characteristicprofile in a short-circuit case and/or overcurrent case, and theshort-circuit and/or overvoltage protection circuit 234 can detect thischaracteristic profile.

Optionally, the control circuit 218 can comprise an active clampingcircuit 236. This can provide a second circuit for a “soft shutdown” inthe event of a fault, which circuit can be used as an alternative to theturn-off circuit 242.

Since an exemplary control circuit for a semiconductor switch having aturn-off circuit 242 described herein and an active clamping circuit 236have been discussed with respect to FIG. 2, an exemplary turn-offcircuit will be explained with respect to FIG. 3.

FIG. 3 shows a driver for the ON state 344, a driver for the OFF state346, a turn-off circuit 342 and a semiconductor switch 304. The drivers344, 346 each include an element having a variable resistance 361, 364(in the example shown in FIG. 3 as an NMOS semiconductor switch, butother switchable semiconductor switches or other elements having avariable resistance can also be used). The elements having a variableresistance 361, 364 are each coupled in series with the optionalresistances 362, 363.

The element having a variable resistance 361 (and the resistance 362) ofthe driver for the ON state 344 are coupled between a control input ofthe semiconductor switch 304 (the gate input in the example shown inFIG. 3) and a first reference potential 360 (V_(DD)). In addition, theelement having a variable resistance 361 is arranged in such a way thatit can receive an ON signal 354 (U_(ON)) of the control circuit at acontrol terminal of the element having a variable resistance 361 (forexample at a gate input of the NMOS 361). If, therefore, thesemiconductor switch is intended to be turned on, a resistance of theelement having a variable resistance 361 is reduced (for example theNMOS semiconductor switch 361 is turned on) so that the control input ofthe semiconductor switch 304 (for example gate terminal of the IGBT 304)is coupled to a potential which is high enough to turn on thesemiconductor switch (for example the first reference potential 360V_(DD) in FIG. 3).

Similarly, the element having a variable resistance 364 (and theresistance 363) of the driver for the OFF state 346 are coupled betweena control input of the semiconductor switch 304 (the gate input of theIGBT in the example shown in FIG. 3) and a second reference potential312. In addition, the element having a variable resistance 364 isarranged in such a way that it can receive a modified OFF signal 356(U_(OFF)*) of the control circuit at a control terminal 364 (shown asthe gate terminal of the NMOS 364). If, therefore, the semiconductorswitch 304 is intended to be switched off, a resistance of the elementhaving a variable resistance 364 is reduced (for example the NMOSsemiconductor switch 364 is turned on) such that the control input ofthe semiconductor switch 304 (gate terminal of the IGBT 304) is coupledto a potential which is low enough for the semiconductor switch to beswitched off (for example the second reference potential 312). Theswitching off of the semiconductor switch 304 by pulling the controlterminal (gate terminal of the IGBT) immediately to the second referencepotential results in relatively rapid decrease in the collector-emittercurrent of the semiconductor switch 304. If, however, a short-circuitcase is present, the rapid decrease in the collector-emitter current asa result of parasitic couplings could result in the generation ofpossibly hazardous overvoltages. In order to prevent the influencethereof, the turn-off circuit 342 can be used in a short-circuit caseand/or overcurrent case to implement soft shutdown.

In the example shown in FIG. 3, a switch 366 (S3) is controlled by afault signal 348 (U_(FLT)) in such a way that the switch 366 (S3) isopen when there is a fault (for example when a corresponding detectioncircuit has identified a fault). As a result, an (original) OFF signal352 (U_(OFF)) is modified by the turn-off circuit 342 in the case of afault. During normal operation, on the other hand, the switch S3 366 isclosed and is coupled to a fixed reference potential 312 so that theturn-off circuit 342 does not influence the element having a variableresistance 364 of the driver for the OFF state 346.

The turn-off circuit 342 includes a circuit for generating acontrol-input reference signal 370 (U_(REF)), a detection circuit 369,which is designed to detect an end of a Miller plateau in a controlinput voltage or in a corresponding control input current of thesemiconductor switch and a first comparison circuit 368 in order tocompare the control-input reference signal 370 (U_(REF)) with thevoltage at the control input of the semiconductor switch 330 (U_(GE)).

First, the circuit for generating a control-input reference signal 370(U_(REF)) is explained. Said circuit includes a capacitance, wherein thereference signal 370 (U_(REF)) is formed by a voltage across thecapacitance. In the example shown in FIG. 3, the capacitance may becoupled to two current sources 372, 373 (I₁, I₂) and can be dischargedthrough current sources 372, 373 (I₁, I₂) in order to reduce a signallevel of the reference signal 370 (U_(REF)) at a first or second rate.The first and second rate may be proportional to the value of thecurrent sources 372, 373 (I₁, I₂) and the capacitance. First, a firstterminal of the capacitance can be coupled to a determined referencepotential 360 (V_(DD)) once the semiconductor switch 304 has been turnedon in response to the ON signal 354 (U_(ON)). This corresponds to thefirst reference potential 360 to which the control input of thesemiconductor switch 304 is coupled to in the ON state, in the exampleshown in FIG. 3. Since a second terminal of the capacitance is at alower potential (this corresponds to the second reference potential 312to which the control input of the semiconductor switch 304 is coupled toin the OFF state, in the example shown in FIG. 3), the capacitance ischarged to the determined voltage during the ON state of thesemiconductor switch 304 and consequently still just prior to thebeginning of the turn-on. As a result, the control-input referencesignal 370 (U_(REF)) “starts” at a predetermined signal level, whichcorresponds to the determined reference potential 360 (V_(DD)).

The first current source 373 is coupled to the second referencepotential 312 via a switch 377 (S1). When the switch 377 (S1) is closed,the first current source 373 discharges the capacitance at the firstrate (corresponding to the current I₁). A control circuit for the switch377 (S1) can be designed in such a way that the switch 377 (S1) is keptclosed from a time at which a short-circuit case and/or overcurrent caseis detected (or a predetermined time span after the detection of ashort-circuit case and/or overcurrent case) up to a time at which avoltage 330 at the control input of the semiconductor switch 304 reachesa voltage which corresponds to a Miller plateau of the semiconductorswitch (shown as signal U_(M) 375). Thus, in this time span, thecapacitance is discharged at a first rate.

In the example shown in FIG. 3, the control circuit for the switch 377(S1) includes a comparison circuit 374, which compares a voltage acrossthe capacitance (i.e. the reference signal 370 (U_(REF))) with athreshold value 375 (U_(M)). This threshold value 375 (U_(M)) can beselected in such a way that it reflects a voltage which corresponds to aMiller plateau of the semiconductor switch. For many IGBTs, this voltagecan be between 9.5 volts and 11.5 volts. If a signal level of thereference signal 370 (U_(REF)) reaches the threshold value 375 (U_(M)),the switch 377 (S1) is opened. The capacitance is now no longerdischarged by the current source 373. This can result in the signallevel of the reference signal 370 (U_(REF)) remaining substantially (forexample with a maximum change of 10% of the initial signal level)constant for a determined time period. This time period is ended by theclosing of switch 376 (S2), and as such a discharge operation of thecapacitance is again initiated by the second current source 372.

This time of the closing of the further switch 376 (S2) can bedetermined by the detection circuit 369, which is designed to detect anend of a Miller plateau in a control input current or a control inputvoltage of the semiconductor switch 304. In the example shown in FIG. 3,an end of the Miller plateau is determined on the basis of a voltage atthe control input of the element having a variable resistance 364 of thedriver for the OFF state 358 (U_(G_OFF)) (for example a gate-voltage ofthe element having a variable resistance 364). If this voltage fallsbelow a predetermined threshold value U_(TH), this can indicate an endof a Miller plateau (the coincidence of the end of the Miller plateauwith the threshold value being undershot cannot be perfect in this case,but the threshold value being undershot provides a good estimation forthe time of the end of the Miller plateau).

An exemplary circuit for generating the threshold value U_(TH) is shownin FIG. 7. In one example, the threshold value U_(TH) is in a range offrom 50% to 150% of an expected gate threshold voltage of the elementhaving a variable resistance 364. In another example, the thresholdvalue U_(TH) can be in a range of between 0.3 and 2 volts. In theexample shown in FIG. 7, the circuit for generating the threshold valueU_(TH) includes a second element having a variable resistance 799 basedon the same technology (an NMOS semiconductor switch in the exampleshown in FIG. 7) as the element having a variable resistance 764 of thedriver for the OFF state. A reference current 785 (I_(REF)) can becoupled to a control input of the second element 799 (for example thegate terminal) and to a first terminal (for example the drain terminal)of the second element 799. An area or a gate width of the second element799 can be K times an area or a gate width of the element having avariable resistance 764 of the driver for the OFF state, wherein thereference current 785 (I_(RF)) is selected such that it is K times athreshold value of the output current of the element having a variableresistance 764 which is designed for the end of the Miller plateau. Inone example, the reference current 785 (I_(REF)) is less than 100microamperes and K is less than 1%. Given such circuitry, the thresholdvalue U_(TH) can correspond to a voltage at the control input of thesecond element having a variable resistance 799.

In the example shown in FIG. 3, a voltage at the control input of theelement having a variable resistance 364 of the driver for the OFF state346 (gate voltage of the NMOS 364) can be used for detecting the time atthe end of the Miller plateau. However, the time can also be determinedon the basis of other signals. In other examples, a current at thecontrol input of the element having a variable resistance 364 of thedriver for the OFF state is used in order to determine an end of theMiller plateau. For example, the current at the control input of theelement having a variable resistance 364 for the driver for the OFFstate 346 can tend towards zero when the end of the Miller plateau isreached. In yet further examples, a control voltage or a control currentof the semiconductor switch 304 can be used. In further examples, acollector-emitter voltage of the semiconductor switch or a switchcurrent of the semiconductor switch can be detected in order to detectan end of the Miller plateau.

By closing the switch 376 (S2), the capacitance, starting from the endof the Miller plateau, is discharged at a second rate. This dischargeoperation can last until the capacitance has been completely discharged(or until the capacitance has been discharged upto a predeterminedminimum value). Therefore, the reference signal 370 (U_(REF)) can havethe profile shown in FIG. 4 with two regions with a falling signallevel. The first and second rates can in this case be set as desired.For example, the second rate can be twice as high or higher than thefirst rate. In other examples, the first and second rates are the same.

The reference signal 370 (U_(REF)) generated by the circuit forgenerating a control-input reference signal shown in FIG. 3 can includetwo regions with a falling signal level, with a region with asubstantially constant signal level being embedded between said regions.This sequence of regions is not essential, however. For example, thefirst and/or second rate can vary in time. In other examples, thereference signal 370 (U_(REF)) can have more than two regions with afalling signal level in which the signal decreases at different rates.Further regions with a substantially constant signal level can beembedded between these regions.

Since the generation of the reference signal 370 (U_(REF)) has beendiscussed in the preceding paragraphs, the use of this reference signal370 (U_(REF)) for achieving a “soft shutdown” will be described below.In this regard, the reference signal 370 (U_(REF)) can be compared witha voltage at the control input of the semiconductor switch 330 (U_(GE))by the first comparison circuit 368. In response to this comparison, amodified control signal 356 (U_(OFF)*) for the element having a variableresistance 364 of the driver for the OFF state 346 can be generated. Inthe example shown in FIG. 3, an OFF signal 352 (U_(OFF)) is convertedinto a modified OFF signal 356 (U_(OFF)*) for this purpose. This cantake place in a subtraction circuit 365.

In this way, the variable resistance of the element having a variableresistance 364 can be adjusted in a closed control loop in order toachieve a profile of the voltage at the control input of thesemiconductor switch 330 (U_(GE)) which corresponds to the profile ofthe reference signal 370 (U_(REF)). In this way, the semiconductorswitch is subjected to “soft shutdown”.

In other examples, the feedback variable for the closed control loop canbe a different signal than the voltage at the control input of thesemiconductor switch 330 (U_(GE)), on the basis of which a turn-offoperation of the semiconductor switch can be monitored (for example acurrent at the control input of the semiconductor switch or acollector-emitter voltage of the semiconductor switch). In these cases,it may be necessary to give the reference signal 370 (U_(REF)) adifferent profile than that shown in the example of FIG. 3. In theseexamples too, however, the manipulated variable of the closed controlloop can be a resistance of the element having a variable resistance 364of the driver for the OFF state 346 (or a control signal which variesthe resistance of the element having a variable resistance 364).

FIG. 4 shows exemplary signal profiles in a system which has a controlcircuit for semiconductor switches having the turn-off circuitsdescribed herein. The signals in FIG. 4 show two exemplary switchingcycles of the semiconductor switch. The first switching cycle (curves onthe left-hand sideof the page) has a normal profile, while ashort-circuit case or overcurrent case occurs in the second switchingcycle (curves on the right-hand side of the page). The first curve showscontrol signals 450 (U_(CMD)), as are transmitted, for example, from adriver interface 226 in FIG. 2 to the control circuit 228. An ON signal452 (U_(ON)) and an OFF signal 454 (U_(OFF)) can be generated from thesecontrol signals 450 (U_(CMD)). The fifth curve shows a profile of acollector-emitter voltage 422 (V_(CE)) of the semiconductor switch and aprofile of a collector-emitter current 423 (I_(CE)) of the semiconductorswitch. Some characteristics of these profiles have already beenexplained with respect to FIG. 1B. At the beginning of the second ONperiod, a fault occurs. The collector-emitter voltage 422 (V_(CE)) doesnot drop as severely as would be expected in the normal case. Inaddition, the collector-emitter current 423 (I_(CE)) increases to ahigher value than in the normal case (an increased fault current 449(I_(FT)) is flowing). This can be detected by a short-circuit and/orovervoltage detection circuit, which thereupon outputs a fault signal448 (U_(FLT)) shown as the sixth curve in FIG. 4. As a result, aturn-off circuit can be activated in order to ensure “soft shutdown”.

For this purpose, a control-input reference signal 470 (U_(REF)) can begenerated. The exemplary reference signal 470 (U_(REF)) in FIG. 4 firstdrops from a predetermined level at a first rate, then remainssubstantially constant for a time period, which is ended by an end of aMiller plateau of the semiconductor switch. Then the signal level of thereference signal 470 (U_(REF)) drops at a second rate. As shown in FIG.4, the turn-off circuit adjusts a gate-emitter voltage 430(U_(GE)-illustrated as the third curve from the bottom) of thesemiconductor switch to the reference signal 470 (U_(REF)) when anovercurrent and/or a short circuit is detected. This can have the resultthat a turn-off operation of the semiconductor switch is extended incomparison with the “hard shutdown” during normal operation. Thisextension can be identified in FIG. 4 by a comparison of the fallingedges of the gate-emitter voltage 430 in the first switching cycle andin the second switching cycle. The delay of the turn-off operation is inthis case connected to a detection of an end of the Miller plateau ofthe semiconductor switch: the semiconductor switch is only ultimatelyturned off when an end of the Miller plateau has been reached. Thus,hazardously high voltages across the load can be avoided.

The fourth curve in FIG. 4 shows an exemplary OFF signal 456 (U_(OFF)*)modified by a turn-off circuit (for example the turn-off circuit shownin FIG. 3). This signal can be fed to a control input of an elementhaving a variable resistance of a driver for the OFF state of thesemiconductor switch in order to adjust the turn-off operation of thesemiconductor switch. In the example shown in FIG. 4, a rising edge ofthe modified OFF signal 456 (U_(OFF)*) is longer than a rising edge ofthe OFF signal 454 (U_(OFF)). This can result in a turn-on operation ofa semiconductor switch in a driver for the OFF state (for example NMOS364 in FIG. 3) and therefore a turn-off operation of the semiconductorswitch (for example IGBT 304 in FIG. 3) being temporally stretched inorder to enable a “soft shutdown”. The lowermost curve in FIG. 4 shows adetection signal 458 (U_(G_OFF)) for a detection circuit 369 which isdesigned to detect an end of a Miller plateau in a control input voltageor in a corresponding control input current of the semiconductor switch.The detection signal 458 (U_(G_OFF)) can correspond to the modified OFFsignal 456 (U_(OFF)*) or be generated on the basis of the modified OFFsignal 456 (U_(OFF)*).

FIG. 6 shows simulated signal profiles for a turn-off circuitcorresponding to the signals in FIG. 4. In particular, it can be seen inthe fourth curve from the top how the profile of a voltage at the gateof the power semiconductor switch (U_(GE)) to be switched is adjusted toa gate reference signal (V_(Gref)) by an exemplary turn-off circuit.

FIG. 5 shows an exemplary turn-off circuit 542 in combination with anactive clamping circuit. The elements of the turn-off circuit and thedriver for the ON state and the OFF state in this case correspond to therespective elements shown in FIG. 3. The turn-off circuit 542 furtherincludes a switch S3 566 which is coupled to the non-inverting input ofcomparator 568 and the capacitance 570 with control-input referencesignal (U_(REF)). When there is no fault detected (fault signal U_(FLT)548 is logic low and the inverted fault signal is logic high), theswitch S3 566 is closed and the voltage at the non-inverting input ofcomparator 568 is substantially equal to return 512. As such, similar toFIG. 3, the modified OFF signal 556 (U_(OFF)*) is substantially the OFFsignal 552 (U_(OFF)). When a fault is detected (fault signal U_(FLT) 548is logic high and switch S4B 576 is on), the control-input referencesignal 570 (U_(REF)) “starts” at a predetermined signal level, whichcorresponds to the determined reference potential 560 (V_(DD)). Thecontrol-input reference signal 570 (U_(REF)) is charged to thedetermined reference potential 560 (V_(DD)) when switch S4A is on (i.e.when U_(ON) 554 is logic high). In addition, the turn-off circuit inFIG. 5 can be deactivated, however, when a corresponding signal 560(U_(ACL)) indicates this. This means, using the example of FIG. 5, thatan active clamping circuit ensures a “soft shutdown” so that theturn-off circuit does not need to become active.

In the example shown in FIG. 5, a signal level of the signal 560(U_(ACL)) of approximately zero means that the active clamping circuitis inactive. If an active clamping circuit is active, the signal 560(U_(ACL)) has a positive or negative level. This state is detected bythe comparison circuits 578, 579 (for example by comparison of thesignal level of the signal 560 (U_(ACL)) with a first (lower) thresholdvalue (E1) and a second (upper) threshold value (E2)). The comparisoncircuits are coupled to a NOR circuit 580 so that it is detected thatthe signal level of the signal 560 (U_(ACL)) is neither above the secondthreshold value (E2) nor below the first threshold value (E1).Thereupon, a corresponding activation signal 577 (SSD_EN) for theturn-off circuit can be generated. For example, an output of the NORcircuit 580 can be coupled to a data input of a D flipflop 581, inresponse to the output of which the activation signal 566 (SSD_EN) isgenerated. The fault signal 548 (U_(FLT)) can in this case be coupled tothe clock input of the D flipflop 581. This is linked to the outputsignal of the first comparison circuit 568 of the turn-off circuit. If,therefore, the activation signal 566 (SSD_EN) has a high signal level,the turn-off circuit engages in the turn-off operation as describedfurther above. If, on the other hand, the activation signal 566 (SSD_EN)has a low signal level (which indicates operation of an active clampingcircuit), the influence of the turn-off circuit is suppressed. In thiscase, the active clamping circuit ensures a “soft shutdown”. Theapparatuses for activating the turn-off apparatus can be providedindependently of the existence of an active clamping circuit. If, in adetermined control circuit, an active clamping circuit is then provided,the apparatuses for activating the turn-off apparatus can havecorresponding circuitry.

In other examples, a selection circuit for selecting between a turn-offcircuit and an active clamping circuit can comprise further elements.For example, a current can be fed to the input for the signal 560(U_(ACL)) prior to each turn-off operation. The additional currentprovided by the internal circuit and sourced to the same net (i.e.U_(ACL) in FIG. 5) may be used to test if a current provided to this netwould lead to a voltage at the net. If that is the case, an activeclamping signal may be effective. In addition, a duration of an outputpulse of the flipflop 581 in response to which the activation signal 566(SSD_EN) is generated can be extended to a predetermined minimumduration. Alternatively, a duration of an output pulse of the comparisoncircuits 578, 579 can be extended to a predetermined minimum duration.With these measures, a susceptibility to faults of the selection circuitcan be reduced. The above description of the examples illustrated of thepresent invention is not intended to be exhaustive or restricted to theexamples. While specific embodiments and examples of the invention aredescribed herein for illustrative purposes, various modifications arepossible without departing from the present invention. The specificexamples of voltage, current, frequency, power, range values, times,etc., are merely illustrative such that the present invention can alsobe implemented with other values for these variables.

These modifications can be implemented using examples of the inventionin the light of the above-detailed description. The terms which are usedin the claims which follow should not be interpreted as restricting theinvention to the specific embodiments which are disclosed in thedescription and claims. The present description and the figures are tobe regarded as illustrative and not as restrictive.

What is claimed is:
 1. A turn-off circuit for a semiconductor switch,wherein the turn-off circuit comprises: an element having a variableresistance, said element being coupled to a control input of thesemiconductor switch; a circuit for generating a control-input referencesignal; a control circuit coupled to adjust a resistance of the elementhaving a variable resistance in response to the control-input referencesignal in a closed control loop in order to turn off the semiconductorswitch; and a detection circuit coupled to detect an end of a Millerplateau in a control input voltage or in a control input current of thesemiconductor switch, wherein the circuit for generating thecontrol-input reference signal is designed to reduce a level of thecontrol-input reference signal in response to a detection of the end ofthe Miller plateau at a predetermined rate, wherein the end of theMiller plateau is detected on a basis of a voltage at the control inputof the element having a variable resistance that falls below apredetermined signal level, and wherein the predetermined signal levelis determined by a reference current and a second element that is basedon a same technology as the element having the variable resistance. 2.The turn-off circuit as claimed in claim 1, wherein the element having avariable resistance is a semiconductor switch.
 3. The turn-off circuitas claimed in claim 2, wherein the element having a variable resistanceis a MOSFET semiconductor switch.
 4. The turn-off circuit as claimed inclaim 1, wherein the variable resistance of the element having thevariable resistance is formed between a drain, anode or collectorterminal and a source, cathode or emitter terminal of the semiconductorswitch.
 5. The turn-off circuit as claimed in claim 1, wherein theelement having the variable resistance is coupled in series with afurther resistance between the control input of the semiconductor switchand a reference potential.
 6. The turn-off circuit as claimed in claim1, wherein the control-input reference signal has a first drop at afirst rate, a region with a substantially constant signal level, and asecond drop at a second rate.
 7. The turn-off circuit as claimed inclaim 6, wherein the second rate is higher than the first rate.
 8. Theturn-off circuit as claimed in claim 6, wherein the first and secondrates vary in time.
 9. The turn-off circuit as claimed in claim 1,wherein the predetermined signal level is in a range of 50% to 150% ofan expected gate threshold voltage of the element having a variableresistance.
 10. The turn-off circuit as claimed in claim 1, wherein thepredetermined signal level is in a range of between 0.3 and 2 volts. 11.The turn-off circuit as claimed in claim 1, wherein an area or a gatewidth of the second element is K times an area or a gate width of theelement having a variable resistance, and wherein the reference currentis selected such that it is K times a threshold value of the outputcurrent of the element having a variable resistance which is designedfor the end of the Miller plateau.
 12. The turn-off circuit as claimedin claim 9, wherein the reference current is coupled to the controlinput, in particular to the gate of the second element having a variableresistance and to the output, in particular to the drain, of the secondelement having a variable resistance, and wherein the reference currentis selected to be less than 100 microamperes and K is selected to beless than 1%.
 13. The turn-off circuit as claimed in claim 1, whereinthe level of the control-input reference signal prior to the end of theMiller plateau being reached is substantially constant, and, in responseto the detection of the end of the Miller plateau, the signal level ofthe control-input reference signal is reduced at the predetermined rate.14. The turn-off circuit as claimed in claim 1, wherein the turn-offcircuit further comprises: a circuit for detecting a voltage at thecontrol input of the semiconductor switch, wherein the control circuitis coupled to adjust the resistance of the element having the variableresistance in response to the control-input reference signal and thevoltage at the control input of the semiconductor switch.
 15. Theturn-off circuit as claimed in claim 14, wherein the control circuitcomprises a first comparison circuit in order to compare thecontrol-input reference signal with the voltage at the control input ofthe semiconductor switch.
 16. The turn-off circuit as claimed in claim15, further comprising: a circuit for generating a control signal forthe element having a variable resistance in response to an output of thefirst comparison circuit, wherein the circuit for generating a controlsignal comprises a second comparison circuit coupled to generate thecontrol signal for the element having the variable resistance inresponse to a comparison of the output of the first comparison circuitwith a fault signal, which indicates a fault state of the semiconductorswitch.
 17. The turn-off circuit as claimed in claim 1, wherein theturn-off circuit is coupled to receive a fault signal, which indicates afault state of the semiconductor switch.
 18. The turn-off circuit asclaimed in claim 17, wherein the fault state of the semiconductor switchis a short-circuit state and/or an overcurrent state.
 19. The turn-offcircuit as claimed in claim 1, wherein the semiconductor switch is apower semiconductor switch.
 20. The turn-off circuit as claimed in claim19, wherein the power semiconductor switch is an IGBT, an IEGT, a powerMOSFET or a power bipolar transistor.
 21. The turn-off circuit asclaimed in claim 1, wherein the turn-off circuit further comprises anactive clamping circuit coupled to actively increase an output voltageof a driver circuit of the semiconductor switch to an extent that isnecessary in order to keep an output voltage of the semiconductor switchbelow a determined threshold voltage, wherein an output of the activeclamping circuit is coupled to a control input of the element having avariable resistance.
 22. The turn-off circuit as claimed in claim 21,wherein the active clamping circuit reduces a voltage at the controlinput of the element having a variable resistance when a voltage betweenthe control input of the element having a variable resistance and asource, collector or cathode input of the semiconductor switchapproaches a predetermined threshold voltage.
 23. The turn-off circuitas claimed in claim 22, wherein coupling of the control-input referencesignal to the control input of the element having the variableresistance is prevented when the active clamping circuit is activated.24. The turn-off circuit as claimed in claim 1, wherein the end of theMiller plateau is detected on a basis of a voltage across the elementhaving a variable resistance, a current at the control input of thesemiconductor switch, a voltage across the semiconductor switch, or acurrent through the semiconductor switch.
 25. The turn-off circuit asclaimed in claim 24, wherein the control circuit is coupled to detectthe resistance of the element having a variable resistance on a basis ofthe control-input reference signal and the voltage across the elementhaving a variable resistance, the current at the control input of thesemiconductor switch, the voltage across the semiconductor switch or thecurrent through the semiconductor switch.
 26. The turn-off circuit asclaimed in claim 1, wherein the circuit for generating a control-inputreference signal comprises two or more current sources and acapacitance, wherein the capacitance is arranged so as to be dischargedfrom the two or more current sources in the event of a fault.
 27. Aturn-off circuit for a semiconductor switch, wherein the turn-offcircuit comprises: an element having a variable resistance, said elementbeing coupled to a control input of the semiconductor switch; a circuitfor generating a control-input reference signal; a control circuitcoupled to adjust a resistance of the element having a variableresistance in response to the control-input reference signal in a closedcontrol loop in order to turn off the semiconductor switch; and adetection circuit coupled to detect an end of a Miller plateau in acontrol input voltage or in a control input current of the semiconductorswitch, wherein the circuit for generating the control-input referencesignal is designed to reduce a level of the control-input referencesignal in response to a detection of the end of the Miller plateau at apredetermined rate, wherein the circuit for generating the control-inputreference signal comprises two or more current sources and acapacitance, wherein the capacitance is arranged so as to be dischargedfrom the two or more current sources in the event of a fault.
 28. Theturn-off circuit as claimed in claim 1, wherein a first of the two ormore current sources is designed to discharge the capacitance after atime at which an end of a Miller plateau has been detected and a secondof the two or more current sources is designed to discharge thecapacitance up to a time at which a threshold voltage is present acrossthe capacitance.
 29. The turn-off circuit as claimed in claim 27,wherein a first of the two or more current sources is designed todischarge the capacitance after a time at which the end of the Millerplateau has been detected and a second of the two or more currentsources is designed to discharge the capacitance up to a time at which athreshold voltage is present across the capacitance.